The present invention relates to a double-tube type Coriolis flow meter with counterbalancing outer tube.
The Coriolis flow meter is designed to measure the mass flow of a flowing medium being measured taking advantage of the fact that when a measuring tube supported at both ends thereof, through which the medium being measured flows, is caused to vibrate, Coriolis force acting on the measuring tube is proportional to the mass flow of the medium being measured.
Now, a conventional double-tube type Coriolis flow meter will be described, referring to FIG. 4. The Coriolis flow meter is of a double-tube type having a hollow cylindrical outer housing 1 with connecting flanges 2 at both ends, in which a double straight-tube assembly comprising a flow tube 4 and an outer tube 5 is disposed coaxially with the outer housing 1. A fluid being measured flows in the straight flow tube 4, and a counterweight 10 is mounted at the center of the hollow straight outer tube 5. Both the flow tube 4 and the outer tube 5 are coaxially secured at both ends thereof to each other via coupling blocks 12, which are rigid bodies. The weight of the counterweight 10 is adjusted so that the natural frequency of the flow tube 4, with the coupling blocks 12 at both ends serving as supporting parts, becomes equal to the natural frequency of the outer tube 5.
At the center of the flow tube 4 and the outer tube 5 provided is a drive unit 7 for causing the flow tube 4 and the outer tube 5 to resonate with each other in opposite phases. A pair of sensors 8 are provided at symmetrical locations on both sides of the drive unit 7 to detect a phase difference produced in the flow tube by the Coriolis force.
This double-tube type Coriolis flow meter is of a simple and compact construction and capable of stably detecting a mass flow proportional to a phase difference produced by the Coriolis force.
In the double-tube type Coriolis flow meter, however, when a change in the temperature of the fluid being measured happens to cause a large temperature difference between the flow tube and the outer tube, thermal stress may be produced in the longitudinal direction of the tubes, causing the spring constant and accordingly. The natural frequency of the tubes to change. This could deteriorate the energy balance, making the resonance of the tubes difficult.
The Coriolis flow meter is usually associated with an error, called the instrumental error, between the true value, namely, the actual flow of a fluid, and the measured value as the result of measurement by an instrument. A commonly practiced method of efficiently correcting the instrumental error in the double-tube type Coriolis flow meter is to measure temperature or stress at a given point on the outer tube and make corrections based on the measurements. But if temperature distribution in the outer tube becomes uneven, accurate error correction cannot be accomplished. The use of coupling blocks 12 that are rigid bodies as used in the conventional type tends to cause an increase in local thermal conduction through the coupling blocks 12 at both ends, rather than even temperature distribution over the overall length, leading to an uneven temperature distribution in the outer tube.
Furthermore, although the vibration supporting ends are generally connected to the tubes by brazing, a large difference between the thermal capacities of the supporting ends and the tubes could make brazing procedures complex, leading to increased cost.
This invention is, therefore intended to solve these problems, and it is an object of this invention to provide a double-tube type Coriolis flow meter wherein the flow tube and the outer tube are connected to one another with sheet members to absorb stresses produced between tubes and limit local thermal conduction to ensure uniform temperature in the outer tube, and the thickness of the sheet members is made closer to that of the tubes to simplify brazing procedures and attain good brazing performances.
This invention was contrived under the aforementioned circumstances, and the double-tube type Coriolis flow meter according to this invention comprises a straight flow tube 4 in which the fluid being measured flows, a hollow outer tube 5 that is disposed concentrically outside the flow tube 4, secured at both ends to act as a counterbalance, a drive unit 7, and a pair of sensors 8. As the flow tube 4 is caused to vibrate by the drive unit 7, a phase difference proportional to the Coriolis force acting on the flow tube 4 is produced by the vibration. The phase difference is detected by the sensors 8 at both ends of the flow tube 4 to measure mass flow. This invention is characterized by a double-tube type Coriolis flow meter in which the flow tube 4 is fixedly fitted to both ends of the outer tube 5 via sheet members.
In this invention, resonance is stably maintained without changing the spring constant due to thermal stress and changing the natural frequency of the tubes since the stress produced between the tubes is absorbed by connecting the flow tube and the outer tube at both ends via sheet members. Furthermore, the use of sheet members helps reduce thermal conduction due to its thin thickness, increasing the ratio of uniform thermal conduction in the longitudinal direction through a space between the flow tube and the outer tube. This contributes to making the temperature of the outer tube uniform. As a result, it is possible to correct the instrumental errors more accurately since temperature measurement at a point on the outer tube to correct instrumental errors becomes more accurate. In addition, by making the thickness of the sheet members closer to that of the tubes, the thermal capacities of them can be made almost equal, leading to simplified brazing procedures and improved brazing performance.
The double-tube type Coriolis flow meter according to this invention is characterized by its construction where the sheet members are formed into a disc shape so as to close the gap between the concentrically disposed flow tube 4 and outer tube 5, with the outer periphery of the flow tube 4 brazed to the inner periphery of the outer tube 5. By forming the sheet members into a disc shape, this invention makes it possible to realize positive brazing with sufficient mechanical strength while maintaining a thin thickness enough to make thermal conduction through the sheet members difficult.
Furthermore, the double-tube type Coriolis flow meter according to this invention is characterized by its construction where the sheet members symmetrically extended toward the outside of the outer tube 5 in the radial direction to form leaf springs 14, with the ends thereof fixedly fitted to the outer housing 1. With this construction, leaf springs 14 of a simple construction can be provided to ensure stable vibration. Forming the coupling plates and the leaf springs into an integral structure helps reduce the number of parts, leading to reduced manufactured cost. Complete agreement of the supporting points of the flow tube and the outer tube with the supporting points of the entire tube assembly ensures stable vibration, leading to a high-performance Coriolis flow meter manufactured at low cost.